Method for preparing a fluoride epitaxial substrate and controlling the crystal growth of perovskite

Abstract

The present application provides a method for preparing a fluoride epitaxial substrate by a solution method, and is used to control the crystallization growth of the inorganic perovskite light-emitting layer CsPbI x Br 3‑x to improve the efficiency of LED devices. The method for preparing the fluoride epitaxial substrate includes: a) injecting an ammonium fluoride diethylene glycol solution into a diethylene glycol solution of a barium salt or a strontium salt and a monovalent oleate to react to form fluoride nanoparticles; b) dispersing the fluoride nanoparticles obtained in step a) in a non-polar solvent, dissolving hydrofluoroboric acid in a polar solvent, and then mixing the two and stirring until the fluoride nanoparticles are dispersed from the non-polar solvent to the polar solvent; c) spin-coating the fluoride nanoparticles dispersed in the polar solvent onto a substrate, and then performing an annealing treatment to prepare a substrate containing a fluoride epitaxial layer, that is, a fluoride epitaxial substrate.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of perovskite light-emitting devices, and particularly relates to a preparation method for improving the performance of LED devices by controlling the crystal growth of a perovskite light-emitting layer by using a fluoride epitaxial substrate that highly matches the perovskite lattice. Background Art

[0002] In recent years, all-inorganic perovskites have attracted much attention due to their excellent charge transport properties, high color purity and thermal stability, and easy solution processability. However, the crystallization process of all-inorganic perovskite CsPbI x Br 3-x thin films is difficult to control, resulting in the formation of uneven thin films, and there are a large number of defects at grain boundaries. These seriously hinder the further development of all-inorganic perovskite CsPbI x Br 3-x for use in light-emitting devices. Currently, the existing reports are to reduce the crystallization temperature and passivate surface defects by doping inorganic ions and adding organic ligands. However, the introduction of a large number of ligands or ions will induce carrier barriers at grain boundaries, reduce the carrier mobility, and cause the efficiency of the devices to remain at a relatively low level.

[0003] Heteroepitaxial growth is one of the most effective methods for preparing semiconductor thin films with low defects and high crystal quality, and has been widely developed in the optoelectronic field. Generally, the crystal growth process of thin films is atomically controlled by an epitaxial substrate, thereby effectively reducing the defects inside the thin films and improving the performance of optoelectronic devices. However, traditional epitaxial growth requires a single-crystal substrate and harsh preparation conditions, such as ultra-high vacuum and temperature. This not only requires high preparation costs, but also limits the universality and scalability of the process. Therefore, a substrate that is conducive to controlling the crystal growth process of perovskite is needed. Summary of the Invention

[0004] Therefore, in order to solve the above problems in the prior art, the present application provides a method for controlling the crystal growth process of a perovskite light-emitting layer by using a fluoride epitaxial substrate that is easily prepared by solution, thereby preparing a high-efficiency LED device. For this purpose, the present application relates to the following aspects:

[0005] <1>. A method for preparing a fluoride epitaxial substrate by a solution method, the method comprising:

[0006] a) Synthesizing fluoride nanoparticles dispersed in a non-polar solvent by a thermal injection method: injecting a diethylene glycol solution containing ammonium fluoride into a diethylene glycol solution of a barium salt or a strontium salt and a monovalent oleate to react to form fluoride nanoparticles;

[0007] b) Disperse the fluoride nanoparticles obtained in step a) in a non-polar solvent, dissolve hydrofluoro boric acid in a polar solvent, and then mix the two and stir until the fluoride nanoparticles are dispersed from the non-polar solvent to the polar solvent;

[0008] c) Spin-coat the fluoride nanoparticles dispersed in the polar solvent onto a substrate, and then perform annealing treatment to prepare a substrate with a fluoride epitaxial layer, that is, a fluoride epitaxial substrate.

[0009] <2>. According to the method described above, wherein the molar ratio of ammonium fluoride: monovalent oleate: barium salt or strontium salt is 2-3: 2-3: 1.

[0010] <3>. According to the method described in any one of the above, wherein the barium salt is a divalent metal salt selected from barium hydroxide octahydrate, barium nitrate, and barium sulfate.

[0011] <4>. According to the method described in any one of the above, wherein the strontium salt is a divalent metal salt selected from barium nitrate, strontium hydroxide, and strontium chloride.

[0012] <5>. According to the method described in any one of the above, wherein the monovalent oleate is selected from potassium oleate and sodium oleate.

[0013] <6>. According to the method described in any one of the above, wherein step a) includes: the temperature of the diethylene glycol solution dissolved with barium salt or strontium salt and monovalent oleate is 100 °C - 140 °C, the diethylene glycol solution containing ammonium fluoride is uniformly injected, and after the reaction, it is quickly cooled with an ice-water bath to avoid particle growth. The obtained fluoride nanoparticles are centrifuged at a rotation speed of 10000 - 12000 revolutions per minute for 3 - 10 minutes, and then dried at a temperature of 90 °C - 120 °C.

[0014] <7>. According to the method described in any one of the above, wherein in step b), the mass ratio of the fluoride nanoparticles to hydrofluoro boric acid is 1: 1.5 - 2.

[0015] <8>. According to the method described in any one of the above, wherein in step b), after the ligand exchange process between the monovalent oleate and hydrofluoro boric acid ends, the upper non-polar solvent is removed, and then the non-polar solution is re-added, and this is repeated several times to remove the oleic acid that may be present in the polar solvent.

[0016] <9>. According to the method described in any one of the above, wherein the annealing is carried out at 80 - 140 °C.

[0017] <10>. A method for controlling the crystallization growth of perovskite, the method comprising:

[0018] d) Use the fluoride epitaxial substrate described in any one of <1> to <8> above as the substrate for spin-coating the perovskite precursor solution, spin-coat the perovskite precursor solution onto the fluoride epitaxial layer of the fluoride epitaxial substrate, and then perform annealing treatment to prepare a light-emitting layer of CsPbI x Br 3-x .

[0019] <11>. A method for preparing a highly efficient light-emitting LED device, the method comprising the following steps:

[0020] d) Use the fluoride epitaxial substrate described in any one of <1> to <8> above as the substrate for spin-coating the perovskite precursor solution, spin-coat the perovskite precursor solution onto the fluoride epitaxial layer of the fluoride epitaxial substrate, and then perform annealing treatment to prepare a light-emitting layer of CsPbI x Br 3-x .

[0021] e) Use the CsPbI x Br 3-x light-emitting layer to prepare an all-inorganic perovskite LED device. Description of the Drawings

[0022] Figure 1 XRD and TEM transmission diagrams before and after ligand exchange of the synthesized barium fluoride nanoparticles provided in Example 1 of the present invention;

[0023] Figure 2 TEM and XRD diagrams of the epitaxial substrate prepared by annealing the barium fluoride nanoparticles provided in Example 1 of the present invention;

[0024] Figure 3 XRD and PL spectrum comparison diagrams of perovskite CsPbI3 formed on a common substrate and on a barium fluoride epitaxial substrate provided in Example 1 of the present invention;

[0025] Figure 4 SEM scanning diagrams of perovskite CsPbI3 formed on a common substrate and on a barium fluoride epitaxial substrate provided in Example 1 of the present invention;

[0026] Figure 5 Device performance comparison diagrams of perovskite CsPbI3 formed on a common substrate and on a barium fluoride epitaxial substrate provided in Example 1 of the present invention;

[0027] Figure 6 XRD and PL spectrum comparison diagrams of perovskite CsPbBrI2 formed on a common substrate and on a barium fluoride epitaxial substrate provided in Example 2 of the present invention;

[0028] Figure 7XRD and PL spectra comparison diagrams of perovskite CsPbBr3 formed on a common substrate and on a strontium fluoride epitaxial substrate provided in Embodiment 3 of the present invention. Detailed implementation manners

[0029] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0030] In a first aspect, the present application provides a method for preparing a fluoride epitaxial substrate by a solution method, the method comprising:

[0031] a) Synthesizing fluoride nanoparticles dispersed in a non-polar solvent by a thermal injection method: injecting a diethylene glycol solution of ammonium fluoride into a diethylene glycol solution of a barium salt or a strontium salt and a monovalent oleate (also referred to as a monovalent metal oleate) to react to form fluoride nanoparticles, and drying the obtained precipitate;

[0032] b) Dispersing the fluoride nanoparticles obtained in step a) in a non-polar solvent, dissolving hydrofluoroboric acid in a polar solvent, and then mixing the two and stirring until the fluoride nanoparticles are dispersed from the non-polar solvent to the polar solvent;

[0033] c) Taking a certain amount of fluoride nanoparticles dispersed in a polar solvent and spin-coating them onto a substrate, and then performing an annealing treatment to prepare a substrate containing a fluoride epitaxial layer, that is, a fluoride epitaxial substrate.

[0034] In the present application, the barium salt refers to a divalent barium metal salt, which can be selected from barium hydroxide octahydrate, barium nitrate or barium sulfate, and the strontium salt refers to a divalent strontium metal salt, which can be selected from barium nitrate, strontium hydroxide or strontium chloride.

[0035] In a second aspect, the present application provides a method for controlling the crystallization growth of perovskite, the method comprising:

[0036] d) Using the fluoride epitaxial substrate obtained in the first aspect as the substrate for spin-coating the perovskite precursor solution, and spin-coating the perovskite precursor solution onto the fluoride epitaxial layer of the fluoride epitaxial substrate, and then performing an annealing treatment at a certain temperature to prepare a light-emitting layer of CsPbI x Br 3-x where 0 ≤ x < 3.

[0037] In a third aspect, the present application provides a method for preparing a highly efficient light-emitting LED device, the method comprising the following steps:

[0038] d) Using the fluoride epitaxial substrate obtained in the first aspect as the substrate for spin-coating the perovskite precursor solution, and spin-coating the perovskite precursor solution onto the fluoride epitaxial layer of the fluoride epitaxial substrate, followed by annealing treatment at a certain temperature to prepare a CsPbI x Br 3-x luminescent layer, where 0 ≤ x ≤ 3

[0039] e) Using the CsPbI x Br 3-x luminescent layer obtained in the second aspect to fabricate an all-inorganic perovskite LED device.

[0040] In a specific embodiment, the present application provides a method for preparing a fluoride epitaxial substrate by a solution method, and provides a method for controlling the crystallization growth process of a perovskite luminescent layer by using the prepared fluoride epitaxial substrate, thereby preparing a high-efficiency LED device, specifically including the following steps:

[0041] a) Synthesizing fluoride nanoparticles dispersed in a non-polar solution by thermal injection: Dissolving a barium salt (or strontium salt) and a monovalent oleate in diethylene glycol, then injecting a certain amount of ammonium fluoride dissolved in diethylene glycol to react to form fluoride nanoparticles, adding deionized water for centrifugation, and washing three times with ethanol and acetone. The obtained precipitate is dried, and the finally obtained particle size is 2 - 7 nm;

[0042] b) Taking a certain amount of the fluoride nanoparticles obtained in step a) and dispersing them in a non-polar solvent, and dissolving a certain amount of hydrofluoro boric acid in a polar solvent, then mixing the two, and stirring at room temperature for about 1 - 10 minutes, such as about 5 minutes (min) for ligand exchange until the fluoride nanoparticles are dispersed from the upper non-polar solvent to the lower polar solvent;

[0043] c) Taking a certain amount of the fluoride nanoparticles dispersed in the polar solvent and spin-coating them onto a substrate, followed by annealing treatment to prepare a fluoride epitaxial layer, obtaining a fluoride nanoparticle thin film with a particle size of 7 - 25 nm;

[0044] d) Using the prepared fluoride epitaxial layer as the substrate for spin-coating a perovskite precursor solution (i.e., a solution prepared by dissolving a ligand, a cesium salt, and a lead salt in a polar solvent at a certain molar ratio to a certain concentration), and then performing annealing treatment at a certain temperature to prepare a CsPbI x Br 3-x luminescent layer;

[0045] e) Using the prepared CsPbI x Br 3-x luminescent layer to fabricate an all-inorganic perovskite LED device, where 0 ≤ x ≤ 3.

[0046] In a preferred embodiment, in step a), the molar ratio of ammonium fluoride: monovalent oleate (potassium oleate or sodium oleate): barium salt (or strontium salt) is 2-3: 2-3: 1, more preferably in the range of 2.5-3: 2.2-2.5: 1. Ammonium fluoride is dispersed in diethylene glycol and completely dissolved at 90 °C - 120 °C. Monovalent oleate and barium salt (or strontium salt) are dissolved in diethylene glycol and heated to 100 °C - 140 °C for complete dissolution. The diethylene glycol solution of ammonium fluoride is injected into the diethylene glycol solvent containing monovalent oleate and barium salt (or strontium salt) at a reaction temperature of 100 - 120 °C.

[0047] In a preferred embodiment, in step a), the diethylene glycol solution containing ammonium fluoride should be uniformly injected using an injection pump, and after the reaction, it should be rapidly cooled in an ice-water bath to avoid particle growth. The rotational speed for centrifugally separating the nanoparticles is 10,000 - 12,000 revolutions per minute, taking 3 - 10 minutes, and the powder drying temperature is 90 °C - 120 °C.

[0048] In a preferred embodiment, in step b), the mass ratio of fluoride nanoparticles to hydrofluoro boric acid is 1: 1.5 - 2. The non-polar solvent can be selected from n-butane, n-hexane, and n-octane, and the polar solvent can be dimethyl sulfoxide or N, N-dimethylformamide. The concentration of fluoride in the non-polar solvents n-butane, n-hexane, or n-octane is, for example, 20 - 80 mg / mL.

[0049] In a preferred embodiment, in step b), after the ligand exchange process between monovalent oleate and hydrofluoro boric acid is completed, the upper non-polar n-butane, n-hexane, or n-octane solvent is removed, and then a new non-polar solvent is added for stirring at room temperature, repeating several times, such as three times, to remove the oleic acid that may be present in polar solvents such as N, N-dimethylformamide or dimethyl sulfoxide.

[0050] In a preferred aspect, in step c), the concentration of the fluoride nanoparticles dispersed in the polar solvent can be, for example, 10 - 40 mg / mL, and the dosage can be, for example, 50 μL - 200 μL. The annealing temperature is 80 - 140 °C, and the annealing time is, for example, 10 - 20 min.

[0051] In a particularly preferred embodiment, in step d), the molar ratio of lead salt (e.g., lead iodide or lead bromide), cesium salt (e.g., cesium iodide or cesium bromide) to ligand (e.g., phenylpropanolamine, 5-aminovaleric acid, or phenethylamine) is 1: 1: 0.3. The concentration in N, N-dimethylformamide or dimethyl sulfoxide is 0.06 - 0.25 mmol / mL, and the annealing temperature is 60 - 80 °C.

[0052] In a preferred embodiment, in the step e), the process for preparing the LED device is as follows: The isopropanol solution of ZnO is spin-coated on the ITO glass substrate at a rotation speed of 2000 - 4000 revolutions per minute to form a film, and annealed at 80 - 150 °C for 10 - 30 min; Next, polyethoxyethyleneimine (1.5 - 3 mg / mL) dissolved in ethylene glycol monomethyl ether solution is spin-coated at a rotation speed of 4000 - 6000 revolutions per minute to form a film, and annealed at 80 - 140 °C for 10 - 20 min; Then, fluoride nanoparticles are spin-coated at a rotation speed of 5000 - 9000 revolutions per minute to form a film, and annealed at 80 - 140 °C for 10 - 20 min; Then, the perovskite precursor solution is spin-coated at a rotation speed of 5000 - 9000 revolutions per minute to form a film and annealed at 60 - 80 °C for 10 min as the light-emitting layer; Finally, MoO x (10 nm) and Al (100 nm) are vacuum-evaporated to prepare the LED light-emitting device.

[0053] Principle and advantages of the present invention:

[0054] The present invention draws on the heteroepitaxial method that can form low-density defects and high-crystallinity quality, and uses fluoride with a high lattice match with perovskite CsPbI x Br 3-x as the epitaxial substrate to control the crystallization growth of the perovskite thin film, form a high-quality thin film, and be used to prepare an efficient LED device. The method for preparing the fluoride epitaxial substrate provided in the method is to synthesize fluoride nanoparticles by the thermal injection method, then perform ligand exchange to remove the oleic acid ligand that hinders charge transport, and then obtain it through spin-coating and annealing. The preparation method is simple, the cost is low, and it has great commercial application value. The fluoride epitaxial substrate prepared by the present invention is composed of small nanoparticles and has a certain ability to reduce leakage current; and due to the high electronegativity of fluoride, it can improve the film-forming property of inorganic perovskite on the organic substrate and facilitate the preparation of high-performance devices.

[0055] The present application provides a method for using a fluoride epitaxial substrate that is easily prepared by solution to control the crystallization growth of the inorganic perovskite light-emitting layer CsPbI x Br 3-x and improve its crystallinity and light-emitting efficiency. The content of the invention includes: using the thermal injection synthesis method to synthesize fluoride nanoparticles in diethylene glycol solution and disperse them in a non-polar solvent; using hydrofluoroboric acid as a ligand for ligand exchange to disperse the fluoride nanoparticles into a polar solvent, and then performing spin-coating and annealing to prepare a fluoride heteroepitaxial layer; using the prepared fluoride epitaxial layer as a substrate for spin-coating and annealing to prepare CsPbI x Br 3-xA high-quality light-emitting layer; finally, the above light-emitting layer is used to prepare an efficient LED device. This method uses a solution-easy-to-prepare fluoride epitaxial substrate to control the crystal growth process of the perovskite light-emitting layer and prepares an inorganic perovskite light-emitting layer with high luminous efficiency. Based on this, the performance of the LED device is significantly improved, and it has the advantages of simple, efficient, and inexpensive preparation processes.

[0056] Example

[0057] The following uses examples to illustrate the present invention in detail. It should be understood that the examples described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0058] In the following examples, unless otherwise specified, the methods used are conventional methods in the art, and the materials, reagents, detection devices, etc. used can be obtained from commercial channels.

[0059] For the preparation process of the LED device: Except for spin-coating ZnO in the external environment, other spin-coatings including evaporation are carried out in a glove box, where the oxygen and water content are both <0.1 ppm, and preferably prepared in a dry inert atmosphere.

[0060] Example 1

[0061] (1) Synthesis of BaF2 nanoparticles and preparation of epitaxial substrate

[0062] Dissolve 3 mmol of barium hydroxide octahydrate and 7 mmol of sodium oleate in 30 mL of diethylene glycol and heat to 100 °C until completely melted, then inject 8 mmol of ammonium fluoride dissolved in 10 mL of diethylene glycol. After the injection is completed, immediately cool down in an ice bath. Subsequently, centrifuge with water at 12,000 revolutions per minute, and then wash with ethanol and acetone. Finally, place it in an oven at 100 °C to dry for later use.

[0063] Take 100 mg of the obtained barium fluoride nanoparticle powder and disperse it in 5 mL of n-octane solvent, and add 5 mL of N,N-dimethylformamide solvent containing 0.1 mL of hydrofluoric acid (concentration 50 mass %), and stir at room temperature for 5 min to complete ligand exchange. According to Appendix Figure 1 a, the XRD of barium fluoride before and after ligand exchange has no change and completely corresponds to the standard card of barium fluoride. The relatively wide full-width at half-maximum indicates that the synthesized barium fluoride has a small particle size. From Appendix Figure 1 b, it can be seen from the transmission diagram that the particle size of the synthesized barium fluoride is about 3.2 nm.

[0064] Take 50 μL of the obtained N,N-dimethylformamide solution of barium fluoride and spin-coat it on the substrate at 8,000 revolutions per minute, and anneal at 100 °C for 10 min. From Appendix Figure 2It can be seen that after annealing, the crystallinity of barium fluoride is enhanced with an obvious orientation of {200}, and the grain size increases from about 3 nm to about 10 nm.

[0065] (2) Preparation of high-quality deep red light-emitting thin films using an epitaxial substrate

[0066] Prepare a solution with a concentration of 0.1 mM / mL by mixing phenylpropanolamine, cesium iodide, and lead iodide in a molar ratio of 0.3∶1∶1, spin-coat it on a barium fluoride substrate at 9000 revolutions per minute, and then anneal it at 70 °C for 10 min. The comparison sample is prepared by spin-coating on a substrate without barium fluoride nanoparticles.

[0067] From the Figure 3 XRD comparison chart in Attachment a, we can see that for the perovskite grown on the barium fluoride nanoparticle substrate, there are only two diffraction peaks located at 14.8° and 28.9° respectively, corresponding to the (110) and (220) crystal planes of the orthorhombic perovskite, and the corresponding peak intensities are high, indicating good crystallinity and orientation. While for the perovskite grown on the substrate without barium fluoride nanoparticles, there are multiple diffraction peaks and the peak intensities are low, with poor crystallinity. At the same time, we compared the luminescence properties of the two. According to Attachment Figure 3 b, for the perovskite grown on the barium fluoride epitaxial substrate, its spectrum is located at 700 nm, and the photoluminescence external quantum yield (PLQY) is 52%, which is significantly higher than that on the substrate without fluoride nanoparticles (21%). The high crystallinity, good grain orientation, and the improvement of PLQY prove the good role of the barium fluoride epitaxial substrate.

[0068] Since the barium fluoride epitaxial substrate is composed of small barium fluoride nanoparticles, these particles can serve as nucleation sites to control the growth of perovskite. By adjusting the concentration of barium fluoride in N,N-dimethylformamide, the growth of perovskite grains can be controlled. According to Attachment Figure 4 , the film of perovskite CsPbI3 grown under the control of the barium fluoride epitaxial substrate is very dense and smooth, while for the perovskite without the barium fluoride substrate, there are phenomena such as inconsistent grain sizes and uneven film surfaces. This situation will lead to leakage in the finally fabricated device, thereby affecting the brightness and efficiency of the device.

[0069] (3) Fabrication and performance testing of LED devices

[0070] The process for fabricating a deep red light-emitting diode (LED) device is as follows: The isopropanol solution of ZnO is spin-coated on an ITO glass substrate at a speed of 4000 revolutions per minute to form a film, and then annealed at 80 °C for 15 minutes. Next, polyethoxyethyleneimine dissolved in ethylene glycol methyl ether solution is spin-coated at a speed of 5000 revolutions per minute to form a film, and then annealed at 100 °C for 10 minutes. Then, BaF2 nanoparticles are spin-coated at a speed of 8000 revolutions per minute to form a film, and then annealed at 100 °C for 10 minutes. Then, the light-emitting layer is spin-coated at a speed of 8000 revolutions per minute to form a film, and annealed at 70 °C for 10 minutes as the light-emitting layer. Finally, vacuum MoO x (7 nm) and Al (100 nm) are used to fabricate the LED light-emitting device.

[0071] Figure 5 This is a comparison chart of the device performance of CsPbI3 perovskite grown on a barium fluoride epitaxial substrate and CsPbI3 perovskite directly grown on the transport layer provided in Example 1 of the present invention. From Figure 5 it can be seen that the external quantum efficiency (EQE = 9.04%) and luminance (923 cd / m 2 ) of the processed CsPbI3 are significantly higher than those of the device fabricated with perovskite CsPbI3 directly grown on the transport layer.

[0072] Example 2

[0073] Phenylpropanolamine, cesium iodide, lead iodide, lead bromide, and cesium bromide are configured into a solution with a concentration of 0.1 mM / mL in a molar ratio of 0.3:0.6:0.6:0.4:0.4, and spin-coated on a barium fluoride substrate at 9000 revolutions per minute, and then annealed at 70 °C for 10 min. The comparative sample is prepared by spin-coating on a PEIE substrate without barium fluoride nanoparticles.

[0074] From the Figure 6 XRD comparison chart in Figure a, we can see that the perovskite grown on the perovskite epitaxial substrate has only two diffraction peaks located at 14.9° and 29.8° respectively, corresponding to the (100) and (200) crystal planes of the cubic perovskite, and the corresponding peak intensities are high, with good crystallinity and orientation. While the perovskite grown on the substrate without barium fluoride has lower peak intensity and poor crystallinity. At the same time, we compared the luminescence properties of the two. According to Figure 6 Figure b, the spectrum of the perovskite grown on the barium fluoride epitaxial substrate is located at 620 nm, and the fluorescence external quantum yield (PLQY) is 47%, which is significantly higher than that (22%) grown on the substrate without barium fluoride nanoparticles. The enhancement of crystallinity and the improvement of PLQY prove the good role of the barium fluoride epitaxial substrate.

[0075] Example 3

[0076] (1) Synthesis of StE2 nanoparticles and preparation of epitaxial substrate

[0077] Dissolve 3 mmol of strontium nitrate and 7 mmol of sodium oleate in 30 mL of diethylene glycol and heat to 100 °C until completely melted. Then inject 8 mmol of ammonium fluoride dissolved in 10 mL of diethylene glycol. Immediately cool down in an ice bath after injection. Subsequently, centrifuge at 12,000 revolutions per minute with water, and then wash with ethanol and acetone. Finally, place in an oven at 100 °C for drying and reserve for use.

[0078] Disperse 100 mg of the obtained strontium fluoride nanoparticle powder in 5 mL of n-octane solvent, and add 5 mL of N,N-dimethylformamide solvent containing 0.1 mL of hydrofluoric acid (concentration 50 mass%). Stir at room temperature for 5 min to complete ligand exchange.

[0079] Take 50 μL of the N,N-dimethylformamide solution of strontium fluoride obtained above and coat it on the substrate at 8,000 revolutions per minute, and anneal at 100 °C for 10 min.

[0080] (2) Preparation of high-quality green light-emitting thin film using epitaxial substrate

[0081] Prepare a solution with a concentration of 0.08 mM / mL by mixing phenylpropanolamine, cesium bromide, and lead bromide in a molar ratio of 0.3:1:1. Spin-coat it on the strontium fluoride substrate at 9,000 revolutions per minute, and then anneal at 70 °C for 10 min to obtain. The comparative sample is prepared by spin-coating on a substrate without strontium fluoride nanoparticles.

[0082] From the XRD comparison chart in Appendix Figure 7 a, we can see that the perovskite grown on the strontium fluoride nanoparticle substrate has only two diffraction peaks located at 15.1° and 30.2° respectively, corresponding to the (100) and (200) crystal planes of cubic perovskite, and the corresponding peak intensities are high, indicating good crystallinity. While the perovskite grown on the substrate without strontium fluoride nanoparticles has lower peak intensity and poor crystallinity. At the same time, we compared the luminescence properties of the two. According to Appendix Figure 7 b, for the perovskite grown on the strontium fluoride epitaxial substrate, its spectrum is located at 520 nm, and the photoluminescence external quantum yield (PLQY) is 62%, which is significantly higher than that on the substrate without fluoride nanoparticles (27%). The enhancement of crystallinity and the improvement of PLQY prove the good role of the strontium fluoride epitaxial substrate.

[0083] Industrial applicability

[0084] It can be seen that the present invention prepares a fluoride nanoparticle epitaxial substrate by a solution processing method at normal pressure and a certain temperature, which can better control the crystallization growth process of perovskite, prepare a film with high luminescence performance and low defects, and then prepare a highly efficient light-emitting LED device. The fluoride nanoparticle epitaxial substrate prepared by the present invention not only serves as an epitaxial growth of perovskite and reduces the leakage current of the device, but also has good stability of the epitaxial substrate and a simple preparation process, and has great industrial application prospects.

Claims

1. A method for preparing a fluoride epitaxial substrate by a solution method, the method comprising: a) Synthesizing fluoride nanoparticles dispersed in a non-polar solvent by a thermal injection method: injecting a diethylene glycol solution containing ammonium fluoride into a diethylene glycol solution of a barium salt or a strontium salt and a monovalent oleate to react to form fluoride nanoparticles; b) Dispersing the fluoride nanoparticles obtained in step a) in a non-polar solvent, dissolving hydrofluoroboric acid in a polar solvent, and then mixing the two and stirring until the fluoride nanoparticles are dispersed from the non-polar solvent to the polar solvent; c) Spin-coating the fluoride nanoparticles dispersed in the polar solvent onto a substrate, and then performing an annealing treatment to prepare a substrate containing a fluoride epitaxial layer, that is, a fluoride epitaxial substrate.

2. The method according to claim 1, wherein the molar ratio of ammonium fluoride: monovalent oleate: barium salt or strontium salt is 2-3: 2-3:

1.

3. The method according to claim 1, wherein the barium salt is a divalent metal salt selected from barium hydroxide octahydrate, barium nitrate and barium sulfate.

4. The method according to claim 1, wherein the strontium salt is a divalent metal salt selected from barium nitrate, strontium hydroxide and strontium chloride.

5. The method according to claim 1, wherein the monovalent oleate is selected from potassium oleate and sodium oleate.

6. The method according to claim 1, wherein said step a) comprises: The temperature of the diethylene glycol solution dissolved with the barium salt or strontium salt and the monovalent oleate is 100 °C to 140 °C, the diethylene glycol solution containing ammonium fluoride is uniformly injected, and after the reaction is completed, it is rapidly cooled with an ice-water bath to avoid particle growth. The obtained fluoride nanoparticles are centrifuged at a speed of 10,000 to 12,000 revolutions per minute for 3 to 10 minutes, and then dried at a temperature of 90 °C to 120 °C.

7. The method according to claim 1, wherein in step b), the mass ratio of the fluoride nanoparticles to hydrofluoroboric acid is 1: 1.5-2.

8. The method according to claim 1, wherein in step b), after the ligand exchange process between the monovalent oleate and hydrofluoroboric acid is completed, the upper non-polar solvent is removed, and then the non-polar solution is re-added, and repeated several times to remove the oleic acid that may be present in the polar solvent.

9. A method for controlling the crystallization growth of perovskite, the method comprising: d) Using the fluoride epitaxial substrate described in any one of claims 1-8 as the substrate for spin-coating the perovskite precursor solution, spin-coating the perovskite precursor solution onto the fluoride epitaxial layer of the fluoride epitaxial substrate, and then performing an annealing treatment to prepare a CsPbI x Br 3-x luminescent layer.

10. A method for preparing a highly efficient light-emitting LED device, the method comprising the following steps: d) Using the fluoride epitaxial substrate described in any one of claims 1-8 as the substrate for spin-coating the perovskite precursor solution, spin-coating the perovskite precursor solution onto the fluoride epitaxial layer of the fluoride epitaxial substrate, and then performing an annealing treatment to prepare a light-emitting layer of CsPbI x Br 3-x ​ e) Using the described CsPbI x Br 3-x luminescent layer to fabricate an all-inorganic perovskite LED device.

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